Ok, sorry I was referring to the eCart 6 checkout wizard, which now does that automatically.
In eCart 5 you would have to update all of the server behaviors on the page. Store Cart Summary in Database, Store Cart Details in Database, Send Email, and Process Transaction.
Update all of them so that instead of the process transaction button press they all use the trigger "Before Page Loads"... then you will have to update the Store Cart Summary in Database and the email body code to refer to the bindings values returned from paypal instead of the hidden form elements that would have been submitted if the page was displayed. (you can use the syntax from the hidden form elements on the page to see how to refer to the associated paypal values)